The present invention relates to a backflush filter device having a filter housing with an inlet and an outlet, which can filter fluid passing through its filter elements and can be cleaned by backflushing.
A backflush filter device is disclosed in WO 98/42426. In this conventional backflush filter, part of the elements employed are conical in shape, particularly in the form of slit screen tube filter elements. Because of the conical construction, the spacing between the individual slit screen tube filter elements or between those elements and cylindrical filter elements is increased. This increased spacing causes the exit stream in the filter housing also to be increased. Consequently, the discharge resistance in filter operation is lowered. The reason for this lowering of the discharge resistance is primarily the relatively larger exit cross-section of the conical filter elements in comparison to cylindrical elements with the filter surface being the same. Since the exit cross-section in the case of conical filter elements is relatively small in comparison to the entrance cross-section formed by the filter surface (that is, the free element surface), a bottleneck is formed in which a large part of the system pressure drops as a function of the magnitude of the flow resistance of the slit tube. Consequently, smaller pressure losses occur. This smaller pressure is more favorable from the viewpoint of energy conservation in the case of backflushing.
In the case of backflushing, a large part of the throughput volume is obtained more or less at the lower end of the filter with conical and cylindrical filter elements. The volume streams then decrease very quickly. Since the conical element is backflushed substantially farther, the velocity gradient is also less steep. An additional cleaning effect in comparison to cylindrical elements is achieved as a result of element conicity when the velocity profiles relative to the filter surface are included. The more or less constant velocity achieved in cleaning the conical filter elements makes the cleaning less harsh, and increases the service life of the filter elements involved.
Preferably, all filter elements are regenerated in succession in backflushing. During filtration of individual elements, filtration by way of the remaining slit tubes continues. The filtration operation is not interrupted at any time. The excess pressure occurring in the backflush filter device allows a small partial stream of the filter element to be cleaned to flow in the opposite direction during the backflush stage, the fouling being separated from the element and carried away. The amount drained in conjunction with backflushing cannot be measured with precision and is based on empirical values. As a general rule large amounts are backflushed over longer periods in order to ensure defouling. Pressure losses occur in backflushing of this nature, and these pressure losses reduce defouling efficiency.
EP-A-0218 034 discloses a device for cleaning porous filter plates which are accessible from both sides and are used for filtration in open basins or channels. These filter plates are cleaned when required by a device which can be moved back and forth. The device has spray nozzles on the side of the clean medium and a suction pipe on the side of the fouled medium. A special vacuum case surrounds the spray nozzles and is under vacuum during operation. The dirt particles stirred up by the spray nozzles are constantly withdrawn; and the clean medium is also not fouled during cleaning operation. The corresponding conventional solution absolutely requires a direct engagement of the porous filter plate involved, so that it is not suitable for use in backflush filter devices.
U.S. Pat. No. 4,462,916 discloses a generic backflush filter device with a suction pump serving as vacuum device for sustaining the backflush. This vacuum device continuously generates a vacuum on the respective filter element to be backflushed, and thus, defouls it. The suction force to be applied by the pump is uniformly generated and depends on the performance of the pump. The impurities drawn off by the hydraulic pump are transferred to a discharge device on its discharge side, one which performs disposal outside the filter device. Because of the continuous pumping operation, the energy cost for defouling of the filter element in question is correspondingly high. In addition, the suction capacity is, as a rule, not adequate for removing especially persistent fouling.
Accordingly, DE-C-196 08 04 discloses pressure pulse backflushing of filters, cross-current filters in particular, by means of a flushing medium. The flushing medium in question is stored in at least one hydraulic accumulator which performs the backflushing by means of a pressure pulse. The respective hydraulic accumulator is connected on its gas side to an operating device which delivers the priming gas pressure on the gas side of the respective hydraulic accumulator. Priming of the accumulator with a fluid, water in particular, delivered by the operating device, is also possible as an alternative. In the case of the conventional pressure pulse backflushing, the fouling to be removed is flushed in the direction of the retained matter side of the filter. This removal is not harmful with cross-current filters, but would not lead to useful results with generic backflush filter devices, since the clean side with the filtrate would then be fouled by the fouling removed from the respective backflushed filter element.
Objects of the present invention are to provide a backflush filter device with intensified backflushing removing even the most persistent fouling, and with a defouling process which is more energy-efficient and more definite.
As a result of a vacuum device having a hydraulic accumulator with an imperious separating element, and of the separating element of the hydraulic accumulator being connected to an operating device for suction or pumping movement, measured defouling with prescribed backflushing amounts takes place. The backflushing is intensified by the vacuum of the vacuum device applied in the form of the hydraulic accumulator. Consequently, as a result of use of the hydraulic accumulator, even with low system pressures and thus with increased energy efficiency, persistent fouling of the active filter surface of the filter element can be removed and discharged from the filter device. Support of backflushing by the hydraulic accumulator is effected for both cylindrical and conical filter elements.
The backflush amount is defined by the volume capacity of the hydraulic accumulator. Through the connection of the separating element to the operating device, the separating element may be actuated so that a pulsed vacuum impact on the filter element to be defouled is generated. This pulsed vacuum impact contributes to removal of persistent fouling, and accordingly, to an increase in defouling efficiency.
In a preferred embodiment of the backflush filter device of the present invention, an actuatable flushing arm is provided for the backflushing. The arm has a fluid discharge for fouled fluid and may be traversed among the free inlet cross-sections of the filter elements in succession. In this way, a filter element may be driven definitely over the flushing arm from other filtration operation with the other filter elements for a backflush process. In addition, the corresponding layout is structurally compact and can be accommodated in one overall filter housing to conserve space. Preferably, the vacuum device is connected to the fluid outlet of the flushing arm.
Especially good results can be achieved in backflushing if a diaphragm accumulator is used as the hydraulic accumulator.
The separating element of the hydraulic accumulator is then connected to the operating device in the form of a working cylinder for suction or pumping movement. In the case of pumping movement, the fouled fluid drawn into the hydraulic accumulator is moved to the exterior, preferably by way of a discharge device. The fluid discharge element is closed off by a sealing element during the discharge process. The vacuum device designed for this purpose may be cost effectively produced.
In a preferred embodiment of the backflush filter device of the present invention, the operating device may be actuated by a valve unit. The sealing element is in the form of a return valve which opens in the direction of the hydraulic accumulator. The operating device may be driven by compressed air, hydraulic means, or electrically.
In another preferred embodiment of the backflush filter of the present invention, the intake volume of the hydraulic accumulator is adapted to the passage volume of the amount of fluid provided for backflushing of the filter element involved. In this manner, complete defouling supported by the vacuum device takes place.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.